The present invention applies to the field of solar cell covers. Such covers comprise at least a substrate and a multilayer bandpass filter and usually also have an ultraviolet reflecting coating and a heat reflecting coating. The cover is cemented to a solar cell, and the multilayer bandpass filter is designed to pass almost all the solar radiation within the band of optical wavelengths to which the cell is sensitive. The transparent cement between the substrate and the cell darkens upon exposure to ultraviolet radiation and must be protected from such exposure. Therefore the substrate is preferably comprised of an ultraviolet absorbing material such as Pilkington's ultraviolet-absorbing CMX glass. The ultraviolet-reflecting coating augments the absorbing action of the substrate, and reduces the rate at which heat is delivered to the substrate by absorption. This coating is usually situated so that it is the first coating encountered by solar radiation traveling toward the underlying solar cell.
The multilayer bandpass filter is designed to reflect the near infrared radiation that lies immediately adjacent the sensitivity band of the solar cell and to be transparent throughout the sensitivity band. This requires that the filter have a wide transmission band that extends from the near infrared to the near ultraviolet. Pioneer work on multilayer bandpass filters for solar cells has been done by Thelen as taught in U.S. Pat. No. 3, 247,392 entitled Optical Coating and Assembly Used as a Band Pass Interference Filter Reflecting in the Ultraviolet and Infrared, U.S. Pat. No. 3, 423,147 entitled Multilayer Filter with a Wide Transmission Band, and U.S. Pat. No. 3,914,023 entitled Wide Band Multilayer Interference Filter.
Thelen's designs meet the above-noted requirements by suppressing higher order reflection bands that fall within the sensitivity band of the cell. Suppression of higher orders is achieved by unique designs in which individual layers are each comprised of material having an index of refraction characteristic of that layer, and where a particular layer may have one of three or more indices. The following material describes the principles underlying Thelen's inventions.
Optical interference filters often have periodic structure. The structure may be illustrated by a sequence of letters such as ABABABABAB where the letter A stands for one quarter wave optical thickness of material A at the design wavelength and the letter B stands for one quarter wave optical thickness of material B at the design wavelength. A short sequence which repeats within a long sequence is called a period. AB is a period within the above sequence and the sequence can be written (AB).sup.5, indicating that the sequence in constructed by repeating the period AB five times.
The sequence (AB).sup.n, where n is any number greater than one, is called a quarter wave stack. A quarter wave stack has the property that it reflects incident radiation at a fundamental or design frequency and at frequencies that are odd multiples of this frequency. The multiples are the orders of the refection. Thus the quarter wave stack has first, third, fifth orders of reflection and an infinite number of other orders corresponding to all the odd numbers. Since the wavelength of the reflected light is inversely proportional to its frequency, the wavelengths of the orders are proportional to the reciprocal of the odd numbers. In a quarter wave stack the frequency of the fundamental or first order is the same as the design frequency.
A quarter wave stack is not suitable for a solar cell cover. If the first order is placed at a position to block near infrared radiation, the resulting third would block visible light at the blue end of the spectrum. Thelen in U.S. Pat. No. 3,247,392 teaches a design that suppresses both the second and third order reflections. His design uses three materials, A, B and C in a sequence with the period ABCBA. In the practice of the invention the indices of refraction of the materials A, B, and C must be selected in accordance with a mathematical formula prescribed by Thelen. Thus, if materials A and B are selected, the index of refraction of C must have the value prescribed by Thelen's formula when the indices of A and B are substituted into it. Thelen found that for coating glass substrates, the materials magnesium fluoride, titanium oxide and lanthanum oxide gave satisfactory results and for quartz substrates, silicon dioxide should be substituted for lanthanum oxide. A third material having an index of approximately 1.96 is then selected.
In U.S. Pat. No. 3,423,147, Thelen teaches the use of three materials in a sequence with a period ABCCBA in order to suppress the second, third and fourth order reflections, thereby achieving a still wider transmission band. In this invention the indices of the materials are related by the formula n.sub.C.sup.2 =n.sub.A .times.n.sub.B.
Thelen describes another wide band filter in which higher order reflections are suppressed in U.S. Pat. No. 3,914,023. This filter uses four materials, A, B, C, and D and the period is ABCDDCBA. Thelen gives a mathematical prescription for selecting the indices of refraction of the four materials and gives specific examples of materials that follow the prescription.
In the first of the patents cited, Thelen teaches the use of his coating on a solar cell cover and claims the use of the coating in this application. He also claims the cover with additional heat rejecting filters and UV rejecting filters, and additional assembly comprising a light-sensitive cell with a coated cover cemented to it.
In addition to his patents, Thelen has published numerous articles in which he describes the principles underlying his filters. See for example Thelen's paper entitled "Multilayer Films with Wide Transmission Bands", J. Opt. Soc. Am., 53,1266. In this paper Thelen describes the theoretical basis for his designs and shows how to design a filter which suppresses selected higher orders.
The coatings of Thelen's inventions which have been cited have the disadvantage that they require more than two materials to be deposited during a coating run. A second, and perhaps more serious disadvantage is that the inventions place constraints on the indices of refraction of the materials to be used in the coating. Materials which satisfy these constraints are not necessarily compatible with the requirements of a given application, so that the coating could not be used for that application.
Rancourt, in U.S. Pat. No. 4,229,066, describes a periodic multilayer coating which provides high reflectance at some IR wavelength (e.g. 10.6 .mu.m) and transmits in the visible. Transmission in the visual is obtained by suppressing those high order reflections which lie in the visual band. The coating uses only two materials. It employs alternating thick H and L layers with 1/4 wave optical thickness at the IR reflectance wavelength. Between these thick layers are AR layer sequences of thin H and L layers. The total optical thickness in the AR stacks is 1/4 wave at the wavelength of transmission (e.g. 0.55 .mu.m).
While Rancourt's invention avoids the use of a third material, the number of layers in a period in Rancourt's invention is 11 compared to 5 in a comparable filter designed according to Thelen's teaching. Thus the use of a material with a selectable index would provide a coating with performance of Rancourt's that would have many fewer layers.
U.S. Pat. No. 5,449,413 by Beauchamp et al. describes a solar cell cover as in Thelen's '392 patent. The cover comprises an ultraviolet reflector, an anti-reflection component and an infrared reflector; Beauchamp's infrared reflector being functionally the same as Thelen's coating for suppressing the second and third orders. The difference between Thelen and Beauchamp lies in the use by Beauchamp of only two materials in the infrared reflecting coating. This eliminates the constraints on indices imposed by Thelen. These constraints preclude the use of materials which are nonabsorptive in the entire spectral range of present day solar cells.
Beauchamp also claims "a means for suppressing low order reflections" as a characteristic of his infrared reflecting filter. The orders that his filter rejects include the second and third order reflections, and these are the same orders that are suppressed by Thelen. Use of the phrase "lower order" to describe the second and third orders as differentiated from the first order is highly unconventional. In any event Beauchamp's use of this terminology does not change the fact that his teaching with regard to order suppression is the same as that of Thelen and therefore has been anticipated by Thelen. Beauchamp is also anticipated by Rancourt, since Rancourt's design provides a means for suppressing higher orders that lie in the visible band.
In U.S. patent application Ser. No. 08/585759, filed Jan. 19,1996, now abandoned Howard, et al., describe a process for sputtering a coating comprised of mixed materials in a "short throw" reactive sputtering process in which substrates are transported past one or more pairs of sputtering targets by a rotating drum. Sputtering and reaction take place within a single continuous reaction zone. FIG. 1 shows a section through a sputtering chamber 15, in which the process described in the application is to be carried out.
The sputtering chamber contains two targets 1 and 2 comprised of different materials such as silicon and niobium which are to be sputtered and at least one plasma generator 3. Substrates 5 are transported by a rotatable drum 4, so that they move in the direction of the arrows 6, transiting the sputtering station 7 and passing through the region 8 in which sputtering and reaction occur. The sputtering targets and the plasma generator are elongated in a direction perpendicular to the plane of the figure so as to cover the full width of the coating and reacting region. The electric power supplies for the sputtering targets are in this embodiment preferably selected from a class of power supplies which generate appropriately timed interruptions or reversals of the voltage being supplied to the targets. Two individual power supplies 9 are individually connected to the sputtering targets, each supply being comprised of a power generation unit 10 and a voltage reversal unit 11. Each power supply contains a feature that enables the power that it delivers to be maintained at a selectable level.
FIG. 1 shows two ducts 12 which represent a system of ducts that is used for the introduction of the working gas, such as argon and a reacting gas such as oxygen. The plasma generator causes an enhanced concentration of reactive species of the reacting gas to occur within the region 8 and thereby facilitates combination of the reacting gas with the deposited material.
When depositing a coating comprised of mixed materials, the power supplies attached to each of the targets are set to deliver amounts of power such that the deposition rates of the material from each of the targets are in the ratio that will produce the desired mixture. A high rate of deposition of the desired coating can be achieved by following the practices described in the application, which allow full compounding of material from both targets without target poisoning.
Using the teaching of the above application a bandpass filter for a solar cell can be fabricated in which all of the layers of mixed material have a different composition and index of refraction. Therefore the indices of these layers can be selected so as to optimize the performance of the filter. An optimized design may be realized using thin film design software which starts with a particular design and then systematically varies the design parameters while computing the performance of the design as it is varied. Prior art optimization varies only the thickness of the layers, whereas in the practice of the present invention both the thickness of the layers and the indices of the mixed layers can be varied.
Use of mixed materials in the multilayer coatings for a solar cell cover provides an increase in the rate of production of the covers in a given sputtering machine over the rate that is achievable when three materials are used. Fabrication using mixed materials as is taught in U.S. patent application Ser. No. 08585759 now abandoned requires that a sputtering station contain two targets, each of which is capable of sputtering a different material. Providing a plurality of such sputtering stations can increase the production rate of a particular sputtering machine so that the rate of deposition of the machine is increased approximately in proportion to the number of stations. In a process involving the deposition of three or more pure materials, a sputtering station would contain targets capable of sputtering each material, so that the station would contain three or more targets. Thus a given station would occupy more space when pure materials are used in the coating than when mixed materials are used. Thus, a larger number of stations can be situated within a given sputtering machine, implying that a high production rate is achievable using mixed materials in a coating than when using pure materials.
Use of mixed materials results in fewer layers for equivalent performance. Fewer layers requires less time for coating because of less lost time due to changeover from one material to another for each consecutive layer.
It is an object of this invention to provide a solar cell cover with a bandpass interference filter, reflecting in the ultraviolet and infrared, which is not subject to the constraints of the Thelen invention.
It is further object of the invention to provide a multilayer stack which implements the Thelen inventions by means of periods comprised of layers, wherein at least one layer has a selectable index.
It is a still further object of this invention to provide a coating in which the index of refraction of each layer of mixed material has a particular selected value, and the value is selected so as to optimize the performance of the filter.
It is still a further object of the invention to reduce the number of sputtering targets within a sputtering station compared to the number required when pure materials are used, thereby enabling a greater increase in production of a particular machine by employing a plurality of stations.
It is a still further object of this invention to reduce the number of layers in a multilayer bandpass interference filter with a give spectral performance.
It is a still further object of this invention to provide a coating for a solar cell in which layers of pure materials and mixed materials are stacked, thereby reducing stress in the coating and achieving enhanced durability of the coating.